Coherent spin field effect transistor

Abstract

A coherent spin field effect transistor is provided by depositing a ferromagnetic base like cobalt on a substrate. A magnetic oxide layer is formed on the cobalt by annealing at temperatures on the order of 1000 K to provide a few monolayer thick layer. Where the gate is cobalt, the resulting magnetic oxide is Co.sub.3O.sub.4 (111). Other magnetic materials and oxides may be employed. A few ML field of graphene is deposited on the cobalt (III) oxide by molecular beam epitaxy, and a source and drain are deposited of base material. The resulting device is scalable, provides high on/off rates, is stable and operable at room temperature and easily fabricated with existing technology.

Claims

1. A method of forming a coherent spin field effect transistor (spin-FET), comprising a cobalt base layer or gate, a cobalt oxide layer overlaying said base layer, a layer of graphene deposited over said magnetic oxide layer, a separated source and drain both in electrical contact with said graphene layer, wherein polarization of ions in layers within said magnetic oxide layer overlaying said base layer and adjacent said graphene layer is ferromagnetic within the layers comprising: (A) forming said gate layer by deposition of said cobalt, followed by annealing to segregate surface oxygen to form a few monolayer thick cobalt oxide layer; and (B) depositing said graphene layer on said cobalt oxide layer by one or more of molecular beam epitaxy (MBE), chemical vapor deposition (CVD) and powder vapor deposition (PVD) and thereafter forming said source and drain on said graphene.

2. The method of claim 1, wherein said gate layer is comprised of cobalt deposited at 750 K in HUV and said gate layer is annealed at 1000 K in HUV to form said magnetic oxide layer.

3. The method of claim 1, wherein said graphene layer is formed by MBE conducted at 1000 K in HUV.

4. A method of forming a coherent spin field effect transistor (spin-FET), comprising a ferromagnetic base layer or gate, a magnetic oxide layer overlaying said base layer, a layer of graphene deposited over said magnetic oxide layer, and an electrical contact with said graphene layer, said method comprising: (A)forming said gate layer of cobalt, (B) forming said magnetic oxide of chrome oxide on said gate layer, (C) depositing said graphene layer over said chrome oxide and annealing said graphene layer, and (D)forming an electrical contact with said annealed graphene.

5. The method of claim 4, wherein said graphene is deposited by molecular beam epitaxy (MBE), chemical vapor deposition (CVD) or plasma vapor deposition (PVD).

6. The method of claim 5, wherein said graphene is deposited using MBE.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention.

(2) FIG. 1 is schematic of a proposed non-local Spin Field Effect Transistor (spin-FET), based on direct graphene growth on a magnetically polarizable oxide.

(3) FIG. 2 is a graphene-based non-local spin valve based on diffusion of individual spin-polarized electrons through the graphene layer. (From Tombros, et al.)

(4) FIG. 3 is schematic illustration of magnetic polaron formation: (a) Alignment of graphene conduction band electrons with Co.sup.+2 ions yields polarization of the conduction electrons by; (b) formation of a magnetic polaron, stabilized by the exchange interactions; and (c) spin-field effect transistor (spin-FET) geometry for testing this effect, which offers a low current, high on/off ratio non-local resistance even at 300 K.

DETAILED DESCRIPTION OF THE INVENTION

(5) Referring initially to the schematic illustration in FIG. 1, and recognizing that this is a conventional representation and that structure and dimensions will be subject to modification depending on the ultimate application envisioned, the formation of each layer of the coherent spin-FET of the invention is described. This is best begun with a discussion of the field itself, as shown.

(6) Graphenethis film must consist of (111)-oriented, sp.sup.2 carbon (graphene), as either a single layer or several layers, as desired to control potential oxide/graphene interactions such as charge transfer [10, 11]. This layer can be deposited by molecular beam epitaxy, or possibly by chemical or physical vapor deposition.

(7) The deposition of graphene on a substrate has been described at some length in U.S. patent application Ser. No. 12/543,053 and U.S. patent application Ser. No. 12/980,763, both of which are included herein-by-reference. Additional advances in the control over few layer graphene deposition are provided in U.S. Provisional Patent

(8) Application Ser. Nos. 61/490,650 and 61/497,971, both of which are incorporated herein-by-reference. The controlled direct growth of graphene by MBE (layer-by-layer growth of macroscopically continuous graphene sheets on Co.sub.3O.sub.4(111) at 1000 K by carbon molecular beam epitaxy (MBE) from a graphite rod source) is described in detail in U.S. Provisional Patent Application Ser. No. 61/522,600. The disclosure of this pending application is incorporated herein-by-reference. Any of the methods described in the incorporated applications can be used to form the graphene field, with a preference for controlled molecular beam epitaxy.

(9) Magnetic Oxide, Source and Drain: This material electrically isolates the graphene from the ferromagnetic gate layer, and allows polarization of the graphene valence/conduction electrons via polarization of the cations in the magnetic oxide. Potential candidates include Co.sub.3O.sub.4(111), Fe.sub.3O.sub.4(111), NiO(111), and potentially spinels such as CoFe.sub.2O.sub.4(111), as well as Cr.sub.2O.sub.3(111), BaFe.sub.2O.sub.4. A critical feature is the polarization within the ion layer adjacent to the graphene (FIG. 1). The polarization of the ions must be ferromagnetic within each layer, even if adjacent ion layers in the oxide are polarized antiferromagnetically to each other. A uniform ferromagnetic polarization within the surface layer is needed to polarize the graphene electrons. Further, the direction of polarization is important. If the oxide ions are polarized in a direction parallel to the surface plane, then the graphene will be similarly polarized, and so must the source and drain. In that case, appropriate source/drain materials could be Co, Ni, Fe, or various alloys. However, if the oxide cations are polarized perpendicularly to the surface plane, then the source and drain should have easy axes of magnetization perpendicular to the plane, and should be made of materials such as Permalloy, CoPd alloys and multilayers, CoPt alloys and multilayers, FePt alloys and multilayers, some L1o ferromagnetic compounds, etc. Similar nickel-iron alloys, and triblends, such as Molybdenum permalloy, may also be used.

(10) Note, since NiO(111) has the same rocksalt structure as MgO(111), deposition of graphene may result in oxide reconstruction, destroying the chemical equivalence of graphene A sites and B sites [12], removing the HOMO/LUMO equivalence at the Dirac point and opening a band gap, as set forth in U.S. patent application Ser. No. 12/980,763. Development of bandgap potential in graphene bearing materials may provide important electronic advantages, in addition to preserving spintronic adaptability.

(11) Ferromagnetic Gate: This layer should have its axis of magnetic polarization that is easily switchable and ideally has a low coercive field for multifunction logic gates, and be ferromagnetic at room temperature. Appropriate materials include Co, Ni, or Fe.

EXAMPLE

(12) In order to prepare a coherent spin-FET of the invention, a sapphire (aluminum oxide (0001)) substrate is prepared for deposition. An electron beam evaporator may be used to reduce movement between chambers and improve productivity, switching in various targets for deposition. Thus, a fifty angstrom layer of cobalt may be deposited under conventional conditions on the substrate at 750 K in UHV. This deposition is followed by an oxidation anneal at 1000 K which results in surface segregation of dissolved oxygen and the development of a thin layer of Co.sub.3O.sub.4 (111) (may be 2-5 ML thick). Graphene (2 or 3 ML) is deposited on the Co.sub.3O.sub.4 using molecular beam epitaxy at 1000 K, yielding a macroscopically continuous graphene film of approximately 3 ML thickness. Graphene may also be deposited via CVD and PVD processes, as disclosed, but MBE is preferred, not only because of the fine control and developed information for this method, but because it is compatible with the other process steps in the formation of the coherent spin-FET of the invention. This leads to high productivity.

(13) In this example, the coherent spin-FET is finished with the application of Co source and drain, and connected in the device for use. On magnetization, it is stable and exhibits a very high on/off rate with low power consumption at temperatures substantially above room temperature. Thus, referring to FIG. 1, the coherent spin-FET prepared has a gate of cobalt, with an insulating layer of cobalt oxide (Co.sub.3O.sub.4). A few ML layer of graphene is deposited over the magnetic oxide, with Co source and drain. Application of a field to the device gives a durable, low power, high on/off rate spin-FET that is operable at room temperature. The resulting coherent spin-FET gives on/off rates (or switching rates) of at least 10.sup.7-10.sup.8 per second depending on the materials and conditions selected.

REFERENCES

(14) [1] H. Haugen, D. Huertas-Hernando, A. Brataas. Spin transport in proximity-induced ferromagentic graphene. Physcal Review B. 77 (2008) 115406. [2] R. Ozwaldowski, I. Zutic, A. G. Petukhov. Magnetism in closed-shell quantum dots: Emergence of magnetic bipolarons. Phys Rev Lett. 106 (2011) 177201. [3] d. R. Yakovlev, W. Ossau, G. Landwehr, R. N. Bicknell-Tassius, A. Waag, S. Schmeusser. Two dimensional exciton magnetic polaron in CdTe/Cd.sub.1-xMn.sub.xTe. Sol St Commun 82 (1992) 29-32. [4] D. R. Yakovlev, W. Ossau, G. Landwehr, R. N. Bicknell-Tassius, A. Waag, S. Schmeusser. Two dimensional exciton magnetic polaron in CdTe/Cd.sub.1-xMn.sub.xTe quantum well structures. Sol St Commun 82 (1992) 29-32. [5] N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, B. J. v. Wees. Electronic spin transport and spin precession in single graphene layers at room temperature. Nature. (2007) 571-574. [6] S. Cho, Y. Chen, M. S. Fuhrer. Gate-tuneable graphene spin valve. Applied Physics Letters. 91 (2007) 123105. [7] W. Han, K. Pi, H. Wang, M. McCreary, Y. Li, W. Bao, P. Wei, J. Shi, C. N. Laun, R. K. Kawakami. Spin transport in graphite and graphene spin valves. Proceedings of SPIE. 7398 (2009) 739819-1. [8] B. Dlubak, P. Seneor, A. Anane, C. Barraud, C. Deranlot, D. Deneuve, B. Servet, R. Mattana, F. Petroff, A. Fert. Are Al.sub.2O.sub.3 and MgO tunnel barriers suitable for spin injection in graphene? Appl Phys Lett. 97 (2010) 092502. [9] V. E. Dorgan, M. Bae, E. Pop. Mobilty and saturation velocity in graphene on SiO.sub.2. Appl Phys Lett. 97 (2010) 082112. [10] J. A. Kelber, S. Gaddam, C. Vamala, S. Eswaran and P. A. Dowben. Proc. SPIE 8100 (2011) 8100 DY 1-13. [11] M. Zhou, F. L. Pasquale, P. A. Dowben, A. Boosalis, M. Schubert, V. Darakchieva, R. Yakimova, J. A. Kelber. Direct graphene growth on Co3O4(111) by molecular beam epitaxy. J Phys: Cond Matt. (2011) in press. [12] S. Gaddam, C. Bjelkevig, S. Ge, K. Fukutani, P. A. Dowben, J. A. Kelber. Direct graphene growth on MgO: Origin of the band gap. J Phys Cond Matt. 23 (2011) 072204.

(15) While the present invention has been disclosed both generically and with reference to specific embodiments and examples, these alternatives are not intended to be limiting unless reflected in the claims set forth below. The invention is limited only by the provisions of the claims, and their equivalents, as would be recognized by one of skill in the art to which this application is directed.